This paper describes the cloning of an 18-kb sequence present in approximately 30 copies on chromosome 17. Most of these are clustered in the region of the breakpoint associated with acute promyelocytic leukemia (APL). These copies map both above and below the breakpoint, and pulsed field gel analysis indicates that the majority of these sequences lie within a region of approximately 2 megabases. The organization of these sequences appears to be that of large imperfect palindromes.